Fix class file for groovy.lang.GroovyObject not found

@rsalama commented on [Sun Dec 16 2018](https://github.com/gradle-guides/creating-multi-project-builds/issues/16)

In order to fix the following error;
```
> Task :greeter:compileJava FAILED
/home/rs/src/gradle/creating-multi-project-builds/greeter/src/main/java/greeter/Greeter.java:5: error: cannot access GroovyObject
final String output = GreetingFormatter.greeting(args[0]);
^
class file for groovy.lang.GroovyObject not found
1 error
```

Add
```
implementation("org.codehaus.groovy:groovy-all:2.5.4")
```
to the dependencies in build.gradle.kts